Researchers at Beijing University of Aeronautics and Astronautics have prepared nearly 1 GPa ultra-high tensile strength aluminum alloy through a combination of high-pressure torsion and powder metallurgy. The strengthening mechanism includes nanocrystalline/subgrain boundary strengthening and nano-scale oxide particles and S/S precipitates secondary phase strengthening. This greatly expands the strength limit of aluminum alloys and provides an effective way to design super-strong aluminum alloys.
Nanocrystalline aluminum alloy production technology can be divided into two categories, including powder metallurgy and other "bottom-up" methods and "top-down" methods represented by severe plastic deformation (SPD). For powder metallurgy methods, nanocrystalline powder can be successfully obtained by mechanical grinding. However, the powder usually needs to be consolidated at high temperature to obtain a dense bulk material, which will result in coarsening of the crystal grains. In the severe plastic deformation process, high pressure torsion (HPT) is one of the most attractive methods of grain refinement because it can provide the maximum applied strain, but further reduction of grain size will be limited.
One promising method to reduce the grain size is to apply high pressure torsion on the metal powder. The oxide layer on the surface of the powder will be broken into dispersed oxide particles through plastic deformation, and then the dynamic recovery during the high-pressure torsion process and the grain growth during the subsequent aging treatment process will be restricted, so as to further refine the grains and increase the hardness or strength. In addition, the additional oxide particles incorporated into the aluminum alloy can act as another reinforcing component, forming a multi-level structure together with grain boundaries and precipitates to further increase strength.
In this study, the strength of age-hardened aluminum alloy was improved by combining powder metallurgy and high-pressure torsion. The paper takes commercial 2024 aluminum alloy, which is widely used as age-hardening aluminum alloy, as an example. The obtained multi-layered nanostructured 2024 aluminum alloy exhibits very high tensile strength, close to 1 GPa. The related paper was published in Materials Science and Engineering A with the title "Achieving ultrahigh tensile strength of 1 GPa in a hierarchical nanostructured 2024 Al alloy".
Paper link:
https://www.sciencedirect.com/science/article/abs/pii/S0921509320306547
In this work, multi-layered nanostructured 2024 aluminum alloy was prepared by high-pressure torsion, and then pre-sintered powder was used for natural aging treatment. The oxide particles introduced by the powder not only refine the Al grains to 115 nm and the sub-grains to 36 nm in the HPT process, but also distribute them evenly in the matrix and serve as the secondary strengthening phase. Subsequent natural aging resulted in 6-16 nm long needle-like S'/S precipitates in the nanocrystals. The multiple strengthening effects including nano-grain/sub-grain boundary hardening and nano-scale oxide particles and S'/S precipitates secondary phase strengthening, resulting in an ultra-high yield strength of 934 MPa, and a tensile strength of 992 MPa.
Studies have shown that nano-grains/sub-grains contain evenly distributed Cu, Mg and O elements. The uniform distribution of the O element shows that the oxide film along the powder boundary is broken into fine particles by high-pressure torsion, and then uniformly distributed in the matrix. This has also been reported in other alloys prepared by the combination of powder metallurgy and high-pressure torsion. Such oxide particles derived from the oxide layer will be mainly aluminum oxide. The fine oxide dispersion incorporated in the initial processing stage will delay recovery during the high-pressure torsion process under high strain, resulting in finer sub-grains. Therefore, in this work, the extremely high-density subgrain boundaries are considered to be produced by the combination of powder metallurgy and high-pressure torsion.